Porter, a law professor and consumer advocate, was declared the narrow winner by the Associated Press on Thursday night in an upscale Orange County district that has been represented by a Republican since 1983.

“I can’t wait to get to work for Orange County and to stand with you 100% of the time!” Porter said in a tweet posted Thursday night that included a video of her standing in front of the U.S. Capitol.

The GOP could face another loss in an area once known as California’s Reagan County. A tally released Thursday night showed Democrat Gil Cisneros taking a narrow lead in the 39th Congressional District over Republican Young Kim in a race to replace the retiring Rep. Edward R. Royce (R-Calif.).

Walters’s loss in the 45th Congressional District was the fifth seat in California that has slipped away from Republicans since last week’s midterm elections.

Nationally, Democrats have gained 36 seats on Republicans, with six races yet to be called.

Porter, a protege of Sen. Elizabeth Warren (D-Mass.), ran on a liberal platform that included advocacy of a single-payer health-care system, a ban on assault weapons and the overturning of the Republican tax cut.

In a tweet Thursday night, Warren wrote: “I’m proud of all of our incredible incoming Democratic House members, but I’m ESPECIALLY proud of our newest: my former student & research partner @katieporteroc! WOO-HOO!”

Walters was easily reelected in the district just two years ago, winning nearly 59 percent of the vote.

John WagnerJohn Wagner is a national reporter who leads The Post's new breaking political news team. He previously covered the Trump White House. During the 2016 presidential election, he focused on the Democratic campaigns of Hillary Clinton, Bernie Sanders and Martin O’Malley. He also chronicled Maryland government for more than a decade. Follow